1. Field of the Invention
The present invention concerns a magnetic resonance apparatus of the type having an integrated gradient and radio frequency coil unit, the gradient and radio frequency coil unit includes at least one antenna rod with a conductor and a gradient coil unit, with at least one electrical connection between the conductor and an electrical contact region located at an adjoining region of the gradient coil unit. The invention also concerns such a gradient and radio frequency coil unit.
2. Description of the Prior Art
Magnetic resonance technology is a known modality to, among other things, acquire images of the inside of a body of an examination subject. Rapidly-switched gradient magnetic fields are superimposed on a static basic magnetic field that is generated by a basic field magnet in a magnetic resonance apparatus (MR apparatus). These rapidly-switched gradient magnetic fields are generated by a gradient coil unit. To excite MR signals, radio frequency signals (RF signals) are radiated into the examination subject with a radio frequency antenna. Integrated gradient and radio frequency coil units are used in order to minimize the space requirement of the gradient coil unit and the radio frequency antenna. These integrated gradient and radio frequency coil units are subject to strong vibrations in operation. This can lead to damaging of components, for example to interruption of a solder contact. This danger exists in particular for the connection of antenna elements to the gradient coil unit or to the radio frequency shield (RF shield) mounted at that location.
In a hollow-cylindrical MR apparatus, the gradient coil unit is fashioned such that a gap or open space in which the antenna rods are arranged exists in the midway along in the axial direction. Radio frequency currents that generate the radio frequency field flow through these connections. The gradient coil units typically are at least partially jacketed by the RF shield. The antenna rods are sealed with the integrated gradient coil unit in a pouring method. In the operation of the MR apparatus, relative to the antenna rod the gradient coil now vibrates with amplitudes in the micrometer range. The movement/vibrations of the antenna rods relative to the RF shield that is sealed with the gradient coil lie in the frequency range of up to approximately 4 kHz given a maximum amplitude (deflection) of about 10 μm in the frequency range around 900 Hz.
The connection of the antenna rods to the RF shield is stressed due to the relative movement. The type of the electrical connection has the purpose of an optimally low-inductivity and low-loss connection. Wide solder bands that are rigidly soldered both on the circuit board and on the shield are used for this purpose. The rigid soldering is also achieved with electrical components (for example an RF choke), these being additionally secured by adhesion.
It is generally necessary for the electrical connection to have an optimally small influence on the antenna properties (no detuning effect, high frequency stability, etc.
A reinforced or rigid connection is most advantageous from an electrical point of view. The connection as part of the antenna influences the tuning of the antenna interim of performance and the resonance frequency. The effective inductivity of the connection (which should be optimally low) as well as the losses in the connection are of decisive importance. Overall the properties of the connection should not be changed by the vibrations or movements.
Such connections exhibit disadvantages. For example, if “thick” copper (for example 1 mm) is used for the connection, the connection at the solder points on the circuit board or the RF shield can crack or break since there only copper thicknesses of 9 μm or 18 μm must be used in order to suppress gradient eddy currents. If “thinner” copper band (0.1 . . . 0.2 mm) is used, cracks can occur within the connection.
Various measures have been proposed to reduce the vibrations in order to be able to maintain such a “rigid” solution. The surfaces of the RF shields are treated in order to improve the connection with the sealing compound [pottant] and to increase the bonding overall. Furthermore, reinforcement anchors (reinforcement ties; stiffening ties) can be installed on the circuit boards/RF shields and additionally through the RF shields—the latter being possible only in a limited manner since the RF shield cannot be permeable. The rigidity of the gradient coil also can be increased by the introduction of glass and glass fiber reinforced plastic rods. All of these measures still do not achieve the desired reliability of the electrical connection.